The black soldier fly (Hermetia illucens Linnaeus 1758) is a fly (Diptera) of the Stratiomyidae family. The adult fly is black, wasp-like and 15-20 mm long (Hardouin et al., 2003). The larvae can reach 27 mm in length, 6 mm in width and weigh up to 220 mg in their last larval stage. They are a dull, whitish color (Diclaro et al., 2009). The larvae can feed quickly, from 25 to 500 mg of fresh matter per larva per day, and with minimal disturbance on a wide range of decaying organic materials, such as rotting fruits and vegetables, coffee bean pulp, distillers' grains, fish offal, corpses (they are used for forensic purposes), and particularly animal manure and human excreta (van Huis et al., 2013; Diener et al., 2011; Hardouin et al., 2003). In ideal conditions, larvae become mature in 2 months, but the larval stage can last up to 4 months when not enough feed is available. At the end of the larval stage (prepupa), the larva empties its digestive tract and stops feeding and moving (Hardouin et al., 2003). The prepupae then migrate in search of a dry and protected pupation site (Diener et al., 2011). The duration of the pupal stage is about 14 days but can be extremely variable and last up to 5 months (Hardouin et al., 2003). The females mate two days after emerging and oviposit into dry cracks and crevices adjacent to a feed source (Diener et al., 2011). The adults do not feed and rely on the fats stored from the larval stage (Diclaro et al., 2009).

Rearing Hermetia illucens has been proposed since the 1990s as an efficient way to dispose of organic wastes, by converting them into a protein-rich and fat-rich biomass suitable for various purposes, including animal feeding for all livestock species, biodiesel and chitin production (van Huis et al., 2013; Diener et al., 2011). The black soldier fly is an extremely resistant species capable of dealing with demanding environmental conditions, such as drought, feed shortage or oxygen deficiency (Diener et al., 2011). One major advantage of Hermetia illucens over other insect species used for biomass production is that the adult does not feed and, therefore, does not require particular care. It is also not a potential carrier of disease. The larvae are sold for pets and fish bait, and they can be easily dried for longer storage (Leclercq, 1997; Veldkamp et al., 2012). A disadvantage of the black soldier fly for biodegradation is that it requires a warm environment, which may be difficult or energy-consuming to sustain in temperate climates. Also, the duration of the life cycle ranges between several weeks to several months, depending on ambient temperature, and the quality and quantity of the diet (Veldkamp et al., 2012). In aquaculture, using feeds based on black soldier fly larvae open additional marketing opportunities for farmers as some customers are opposed to the use of fishmeal in aquaculture feeds (Tiu, 2012).

Several methods for rearing black soldier flies on substrates such as pig manure (Newton et al., 2005), poultry manure (Sheppard et al., 1994), and food wastes (Barry, 2004) have been designed. Rearing facilities use the migrating behaviour of the prepupae for self-collection: larvae climb up a ramp out of a rimmed container to eventually end in a collecting vessel attached to the end of the ramp (Diener et al., 2011). Optimum conditions include a narrow range of temperature and humidity, as well as a range of suitable levels of texture, viscosity, and moisture content of the diet. Temperature should be maintained between 29 and 31ºC, though wider ranges may be feasible. Relative humidity should fall between 50 and 70%. Higher relative humidity makes the diet too wet, and more generally the diet should have enough structure, otherwise the larvae may have a difficult time crawling on it, consuming it and getting an adequate oxygen supply (Barry, 2004).

It is also necessary to maintain a year-round breeding adult colony in a greenhouse with access to full natural light. The greenhouse must be a minimum of 66 m3 to allow for the aerial mating process (Barry, 2004). Ranges of optimal temperatures, for mating and ovipositing, of 24-40°C or 27.5-37.5°C have been reported (Sheppard et al., 2002). Wide ranges of relative humidity are tolerated: e.g. 30-90% (Sheppard et al., 2002), or 50-90% (Barry, 2004). The greenhouse will need a container with a very attractive, moist medium to attract egg-laying female adults (Barry, 2004).

Distribution

Hermetia illucens is native from the tropical, subtropical and warm temperate zones of America. The development of international transportation since the 1940s resulted in its naturalization in many regions of the world (Leclercq, 1997). It is now widespread in tropical and warmer temperate regions between about 45°N and 40°S (Diener et al., 2011).

Processes

Black soldier fly larvae are used live, chopped or dried and ground. There have been attempts to create a defatted meal by cutting the larvae to enable the leakage of intracellular fat and then transferring the material to a tincture press (Kroeckel et al., 2012).

Environmental impact

The black soldier fly can be used commercially to solve a number of environmental problems associated with manure and other organic wastes. Adult flies are not attracted to human habitats or foods and not considered a nuisance (van Huis et al., 2013).

Biomass conversion

Dense populations of larvae can convert large volumes of organic waste into valuable biomass (van Huis et al., 2013). For instance, larvae can reduce the accumulation of manure from laying hens and pigs by 50% or more without extra facilities or added energy (Sheppard et al., 1994; Newton et al., 2005; Barry, 2004). In Costa Rica reduction values of 65-75% have been observed in field trials with household waste (Diener et al., 2011). In confined bovine facilities, the larvae were found to reduce available phosphorous by 61-70% and nitrogen by 30-50% (Newton et al., 2008).

Housefly control

Black soldier fly larvae are a competitor to housefly larvae (Musca domestica), as they make manure more liquid and thus less suitable for housefly larvae. Their presence is also believed to inhibit ovipositing by the housefly. For instance, they have been shown to reduce the housefly population of pig or poultry manure by 94-100%. As a result, they can help to control housefly populations in livestock farms and in households with poor sanitation, thereby improving the health status of animals and people since the housefly is a major vector of disease (Sheppard et al., 1994; Newton et al., 2005).

Low pathogenicity

Unlike other fly species, Hermetia illucens is not a disease vector: not only the eggs are never laid on decaying organic material, but, since the adult fly cannot eat due to its lack of functioning mouthparts, it does not come in contact with unsanitary waste materials. Additionally, the larvae modify the microflora of manure, potentially reducing harmful bacteria such as Escherichia coli 0157:H7 and Salmonella enterica (van Huis et al., 2013). It has been suggested that the larvae contain natural antibiotics (Newton et al., 2008).

The fatty acid composition of the larvae depends on the fatty acid composition of the diet. The lipids of larvae fed cow manure contained 21% of lauric acid, 16% of palmitic acid, 32% of oleic acid and 0.2% of omega-3 fatty acids while those proportions were 43%, 11%, 12% and 3%, respectively, for larvae fed 50% fish offal and 50% cow manure. Total lipid content also increased from 21% to 30% DM. Feeding black soldier fly larvae with a diet made of wastes containing desirable omega-3 fatty acids is, therefore, a way to enrich the final biomass (St-Hilaire et al., 2007b).

Pigs

Black soldier larvae meal was found to be a suitable ingredient in growing pig diets, being especially valuable for its amino acid, lipid and calcium contents. However, its large ash content and its relative deficiency in methionine, cysteine and threonine require that it is included in a balanced diet. It was as palatable to pigs as a soybean meal based diet (Newton et al., 1977).

Dried black soldier fly prepupae meal was fed to early weaned pigs as a replacement (0, 50, or 100%) for dried plasma (5% during a first phase, 2.5% during a second phase, and 0% during a third phase), with or without amino acid supplementation. Without amino acid supplementation, the 50% diet gave slightly better performance during phase 1 (4% extra gain, 9% increase in feed efficiency). However, the 100% diets did not perform as well as the control and overall performance was reduced by 3 to 13%. Additional refinement (cuticle removal and rendering) may be necessary to make prepupae black soldier fly suitable for early weaned pigs (Newton et al., 2005).

Poultry

As a component of a complete diet, black soldier fly larvae meal has been found to support good growth in chicks. Chicks fed a diet containing dried black soldier fly larvae as the protein supplement gained weight at a rate of 96% that of chicks fed soybean meal plus fat (non-significant), but they only consumed 93% as much feed (significant) (Hale, 1973).

Fish

Several experiments have shown that prepupae black soldier fly could be a partial or full substitute for fish meal in fish diets. However, additional trials as well as economic analyses are still necessary as reduced performance has been observed in some cases. The type of rearing substrate and the processing method affect the utilization of the larvae by fish.

Channel catfish (Ictalurus punctatus)

Chopped soldier fly larvae grown on hen manure fed to channel catfish alone or in combination with commercial diets resulted in a similar performance (body weight and total length) as the control diets. Aroma and textures of channel catfish fed larvae were acceptable to the consumer (Bondari et al., 1981). A later study was less favourable: replacement of 10% fish meal with 10% dried soldier fly larvae resulted in slower growth over a 15-week period for subadult channel catfish grown in cages. However, the substitution did not reduce growth significantly when channel catfish were grown in culture tanks at a slower growth rate. Feeding 100% larvae did not provide sufficient dry matter or protein intake for channel catfish grown in tanks to allow a sufficient growth. Chopping of the larvae improved weight gain and efficiency of utilization (Bondari et al., 1987). A comparison between menhaden fish meal and black soldier fly prepupae meal showed that the latter could be advantageous as a replacement for fish meal, but that an inclusion rate higher than 7.5% was unnecessary (Newton et al., 2005).

Blue tilapia (Oreochromis aureus)

Chopped black soldier fly larvae grown on hen manure fed to blue tilapia alone or in combination with commercial diets resulted in a similar performance (body weight and total length) as the control diets. Aroma and texture of tilapia fed larvae were acceptable to the consumer (Bondari et al., 1981). In a later experiment, feeding 100% dry larvae did not provide sufficient dry matter or protein intake for tilapia grown in tanks to allow a sufficient growth. Chopping of the larvae improved weight gain and efficiency of utilization (Bondari et al., 1987).

Alligator (Alligator mississippiensis Daudin)

Dried black soldier fly pupae, reared on food waste, and fed to juvenile alligators in a 3-month trial were less well accepted than a commercial feed and, therefore, not recommended, even though they supported growth (Bodri et al., 2007).

Mountain chicken frogs (Leptodactylus fallax)

Black soldier fly larvae can supply high levels of dietary minerals without a need for additional Ca, provided target species "chew" their food or worms are processed in a way to break the exoskeleton (Dierenfeld et al., 2008).